This invention generally relates to reaction wheels used to provide attitude control for spacecraft, and more specifically applies to reaction wheel arrays.
Reaction wheels are commonly used to provide attitude control for a variety of spacecraft. Reaction wheels typically comprise a rotor, bearings and motor, with a spin direction of the reaction wheel fixed within the vehicle structure. The motor provides the ability to vary the wheel speed of the rotor. As the rotor speed is varied, a momentum exchange occurs and the motor provides a torque on the vehicle about the spin axis
In most applications, multiple reaction wheels are used in a reaction wheel array. The multiple reaction wheels in the array are arranged so that their spin axes span three dimensions for three axis control. Arranging the multiple reaction wheels in this way allows the array to apply torque to the vehicle along different axes, generally all three. Torque can be selectively applied to these axes to provide attitude control of the vehicle.
A typically array includes at least three reaction wheels. This number is the minimum that would enable the array to apply torque to the vehicle in any direction. In many applications, arrays include more than three reaction wheels. These additional reaction wheels enhance the array""s torque and momentum capacity and/or provide backup in the case of a failure in one of the other reaction wheels.
Imperfect rotation of a reaction wheel""s rotor can sometimes cause disturbances. These disturbances, typically in the form of vibrations at harmonics of the spin, notation and precession frequencies, can interfere with the performance of the vehicle. For example, vibration in a satellite may prevent the satellite or its payload from accurately fixing on a desired target.
The vibration and the effect of the vibration on the satellite generally change with the rotational frequency of the rotors. In some frequency ranges the vibrations caused by the rotation of the rotor may be within acceptable limits, while in other frequency ranges the vibrations may exceed acceptable limits. Wheel speeds typically differ among the various wheels in an array, and these speeds change with time. Because the spacecraft dynamics include flexible structures, which respond differentlyxe2x80x94even resonatexe2x80x94at various frequencies, the array""s vibration interacts with the spacecraft dynamics in a way that depends on the wheels""spin speeds. Additionally, the effects of the multiple different rotors may be additive to cause different combinations of frequency ranges to be problematic.
In addition to causing disturbances, the operation of the reaction wheel array can have other frequency-dependent costs. For example, the energy required to effect attitude control of a vehicle can depend upon the combination of rotational frequencies used because the electromagnetic power required to torque a wheel increases with its spin speed. Another example is the thermal effect of operating a wheel at high speed: without explicit attention, wheel speeds can be needlessly high, introducing unwanted and unnecessary heat. Again, these costs can be acceptable in some situations and unacceptable in other.
Thus, what is needed is an improved system and method for operation of reaction wheels that minimizes the cost of operating these wheels, including minimizing the effects of frequency induced disturbances on he vehicle.
The present invention provides a momentum control system and method that provides attitude control for a vehicle while minimizing the negative effects of the momentum control system. The momentum control system includes a plurality of reaction wheels, with each reaction wheel including a rotor that rotates about an axis. The momentum control system varies the rotational speed of the rotor to provide torque to the vehicle, effecting an attitude or orientation change in the vehicle.
The momentum control system and method include at least one more reaction wheel than the degrees of freedom under control. For example, in a vehicle designed to rotate in all three directions, at least four reaction wheels would be provided. The additional reaction wheel provides an additional control parameter that can be used to minimize the cost of the momentum control system""s performance. The cost of the momentum control system that can be minimized includes the effects of vibration, power consumption, and undesirable changes in rotational direction, among others.
The momentum control system and method minimizes the cost of the system by selecting rotational frequency combinations for the reaction wheels that reduce the negative effects of the wheels while still providing the desired torque to the vehicle. Stated another way, the system and method uses the flexibility provided by the additional reaction wheel and actively selects and uses rotational frequency combinations that provide the required torque while also avoiding, where possible, rotational frequencies that cause excessive cost to the system.
The additional reaction wheels used to provide the rotational frequency flexibility can comprise additional reaction wheels added for this purpose or the use of reaction wheels that also serve as backups in the case of failure.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.